1. Field of Invention
This invention relates to a hot-melt adhesive, particularly an adhesive made from a copolymer of ethylene and carboxylic acid blended with a synthetic wax such as a Fischer-Tropsch wax and a high-melt index ethylene/unsaturated carboxylic acid wax having a melt index of at least about 5000 dg/min. The adhesive is particularly useful in rod or strip form in direct, through-feed adhesive dispensers used for application of shoe last adhesive.
2. Background Discussion and Related Art
Direct, through-feed adhesive melting and applying systems have long been used in the manufacturing of shoes. These are systems in which solid adhesive material is supplied to a dispensing apparatus at the rate at which heat softened adhesive is applied to a workpiece and in which there is no reservoir or other substantial body of adhesive held in molten condition out of the direct line of flow from the entry to the exit of the dispensing apparatus. The adhesive for these systems are in the form of slender, flexible rods or stands. These rods must be sufficiently stiff to be fed longitudinally but adequately flexible to be coiled and uncoiled for use without cracking and without blocking or adhering to adjacent rod when coiled. To work effectively, the adhesive rod should have a combination of properties, particularly freedom from tack, dimensional stability, shape retention and resistance to degradation during storage so that the rod doesn't change from the shape required for cooperation with the softening and dispensing passageway.
U.S. Pat. Nos. 3,283,890 and 3,317,368, which are incorporated herein by reference, teach such systems. U.S. Pat. Nos. 2,762,716 and 2,765,768 are cited in U.S. Pat. No. 3,283,890 as showing direct through-feed systems. U.S. Pat. No. 3,283,890 teaches rods of substantially uniform cross-section made by blending various ingredients. U.S. Pat. No. 3,317,368 teaches a composite thermoplastic adhesive rod having a longitudinally extending central portion and an external layer surrounding the central portion.
Polyamide rods have typically been used with direct through-feed systems for shoe last adhesives. The polyamides, however, tend to become brittle at low temperatures and have poor thermal stability.
It has been found that the polyamide rods for use on existing through-feed systems as shoe-last adhesives can be replaced with hot melt adhesives based on copolymers of ethylene and an .alpha., .beta.-ethylenically unsaturated carboxylic acid if a combination of low viscosity, high stiffness, good elongation, fast set speed on a shoe line, and specific adhesion to a broad range of substrates (e.g., oily leather, synthetic leather, shoe board, and polyvinyl chloride) can be obtained. The ethylene copolymers alone do not meet these requirements. It has further been found that through proper selection of ethylene copolymers and by blending them with certain waxes, a shoe last adhesive that will run effectively on existing through-feed systems in place of higher cost polyamides results.